It is well known to the person skilled in the art that molecules of nematic phases (xe2x80x9cnematicsxe2x80x9d) and liquid crystals in general, on coming into contact with a boundary surface, orient themselves in one or more directions because of their interaction with the substrate.
For example, on a solid surface that has been rubbed in one direction, the mesogenic molecules generally orient themselves in a direction close to the rubbing direction. This orientation is said to be xe2x80x9cmonostablexe2x80x9d and is characterized by a single xe2x80x9ceasyxe2x80x9d direction for the nematic, usually defined by the angles xcex80 and xcfx860 (see FIG. 1): the zenith angle xcex80 between the easy axis f and the normal z to the substrate; and the azimuth angle xcfx860 between the easy axis f and a selected fixed direction in the plane of the substrate.
The easy axis f corresponds to a minimum of the interaction energy between the nematic and the boundary phase.
By applying an external field, it is possible to reorient the nematic on the surface. The surface energy increases, and this additional energy, due to the deflection of the surface director ns relative to the easy axis f is called the anchoring energy W and it is a function of the angles xcex8 and xcfx86 that define ns:
W=W(xcex8, xcfx86)
In the case of monostable anchoring, the anchoring energy W has a single minimum which corresponds to a single easy axis (generally no distinction is made between the direction ns and the opposite direction xe2x88x92ns, because the nematic phase is non-polar). A large number of treatments (see [1]) give this simple anchoring, which is in very widespread use in liquid crystal display devices.
Nevertheless, there exist anchorings having a plurality of energy minima W and thus having a plurality of easy directions. For example, on layers of SiO evaporated in a vacuum under certain conditions, the orientation of the nematics is bistable, with two easy directions f1 and f2 (see FIG. 2) defined by xcex802=xcex801 and xcfx8602=xe2x88x92xcfx8601. Several devices have been proposed and made using bistable anchoring of nematics. See documents [2-5].
Another well known class of anchorings comprises degenerate anchorings. Under such circumstances, there exists an entire continuum of easy directions, corresponding to the same zenith angle xcex80 and an arbitrary azimuth angle. The anchoring energy in this case is a function of xcex8 and does not depend on xcfx86:
W=W(xcex8)
In this case it can be said that azimuth anchoring does not exist or that it is infinitely soft.
Depending on the value of xcex80, it is possible to distinguish planar degenerate anchoring (xcex80=90xc2x0) with ns parallel to the surface, or conical degenerate anchoring (0 less than xcex80 less than 90xc2x0) with ns free to rotate on a cone of aperture angle 2xcex80 (see FIG. 3). Another special case corresponds to xcex80=0 (homeotropic anchoring) which gives a monostable anchoring effect with the molecules perpendicular to the surface (xcfx86 is not defined when xcex8=0 )
Degenerate anchorings are typical on a plane surface between the nematic and an isotropic phase. Under such circumstances, there is nothing to impose an azimuth direction, and by symmetry the anchoring energy minimum is achieved for all possible angles xcfx86 (0xe2x89xa6xcfx86 less than 360xc2x0).
Experimentally, degenerate anchoring has often been observed on the free surfaces of nematic drops or on nematic-liquid interfaces. This type of anchoring has been studied from an academic point of view, but has not at present found any application because of its unstable nature: the liquid-liquid or liquid-gas interface is very easy to deform, it creates faults easily and it is difficult to apply an electric field through the interface.
Descriptions of the prior art relating to degenerate anchorings are to be found in document [6].
From studies performed by the inventors, it results in principle that degenerate anchoring can be obtained on any isotropic solid surface, e.g. inorganic or organic glasses, but that in reality such anchoring is rarely observed because two main phenomena intervene.
The first of these phenomena identified by the inventors corresponds to adsorption of mesogenic molecules on the surface.
On first contact between the nematic and the substrate, e.g. while the cell is being filled, no azimuth direction is imposed and the orientation of the molecules is defined by chance or by the flow. Initial orientation is thus usually very inhomogeneous, with xcex8=xcex80 and arbitary xcfx86, but with xcfx86 depending on position on the substrate. Very quickly, the nematic molecules in direct contact with the surface are adsorbed onto the substrate. As a result their order and their orientations are memorized on the surface and they are imposed on the nematic molecules that remain in the volume close to the substrate. Although adsorption is theoretically a reversible process, in practice, the characteristic times for desorption at ambient temperature are very long (days or even years). The orientation of the sample thus remains poorly defined, non-uniform, and strongly anchored to the substrate.
The second of these phenomena identified by the inventors corresponds to the nematic orienting the substrate.
With relatively soft substrates, e.g. polymer layers, a second phenomenon can occur. Even though adsorption is low, the interaction of the substrate with the nematic can make it anisotropic, e.g. by locally orienting the polymer chains. In this case also, the ill-defined and non-uniform initial state is memorized on the substrate, and this is accompanied by azimuth anchoring energy that destroys the degenerate anchoring.
Due to these two orientation memory phenomena, it would appear to be difficult to produce and use degenerate anchoring on solid surfaces. That is why, so far, degenerate anchoring has not found any application.
An object of the present invention is to improve liquid crystal devices to enable degenerate or nearly degenerate anchoring to be used.
More precisely, an object of the present invention is to propose novel means enabling degenerate or nearly degenerate anchoring to be obtained on solid substrates, without liquid crystal memory, thus enabling such anchoring to be used in display devices.
In the context of the present invention, these objects are achieved by a liquid crystal display device comprising a liquid crystal material sandwiched between two confinement plates, the device being characterized by the fact that at least one of the plates is provided with treatment which defines degenerate azimuth anchoring without azimuth orientation memory.
According to another characteristic of the present invention, the treatment is treatment for passivating the surface of at least one of the plates by inhibiting adsorption sites on said surface.
The treatment may be treatment that operates by saturating adsorption sites.
According to another advantageous characteristic of the present invention, the treatment includes a coating comprising a polymer which includes chains that are fluid or very mobile, or indeed are self-lubricating, i.e. lacking any sites capable of adsorbing the liquid crystal.